Securing new energy sources is emerging as an international issue, due to the recent problems of depletion of fossil fuels and global warming. Accordingly, development of renewable energy sources and energy storage to enhance the efficient use of energy are becoming increasingly important.
Particularly, in the automobile industry, development of electric vehicles is urgently required, due to the coming depletion of fossil fuels. However, in the case of using conventional lithium secondary batteries, driving a vehicle over relatively long distances of more than 200 km on a single battery charge may be problematic. Conventional lithium secondary batteries are also not appropriate for long-term energy storage systems storing power generated by renewable energy sources.
In order to resolve this problem, development of new high-capacity, high-output materials and of new technological designs is necessary. In particular, the development of novel anode materials has been receiving a great deal of attention, due to limitations on the development of cathode materials.
As an anode material for a secondary battery, graphite-based materials may be used. However, using such graphite-based materials may result in relatively low capacity (theoretical capacity: approximately 372 mAh/g, approximately 830 mAh/ml; practical capacity: approximately 330 mAh/g), and therefore, development of an anode material having a capacity of 500 mAh/g or above is becoming increasingly important, in order to replace graphite-based materials in preparing high-capacity lithium secondary batteries.
Silicon (Si)-based materials are emerging as new anode materials that can replace graphite-based materials. The biggest advantage of secondary batteries using such Si-based materials is a large capacity, around 4 times the unit volume and 10 times the unit mass of secondary batteries using graphite-based materials. Also, in case of forming such batteries with LiCoO2, LiMn2O4, and the like, the voltage of the batteries is approximately 3.4V, merely 0.3V lower than 3.7V of the conventional batteries. Thus, another advantage of such secondary batteries is an immediate possible commercialization if the anode materials are made stable for charging and discharging. However, a problem with the Si-based materials as anode materials may also exist regarding weight and volume after a reaction with lithium, since the Si-based materials show a great change in volumes after reaction with lithium, whereas the change in the volumes of graphite-based materials is small after reaction with lithium.
As a method of avoiding the volumetric expansion of Si materials after reaction with lithium (Li), Sanyo has developed a method using Si-based materials having a columnar structure as electrodes, the volume of which can be expanded by controlling the form thereof. The method is known to include forming the electrodes by applying Si columnar structure onto a current collector substrate through physical deposition. The Si-based materials in the columnar structure in this method reportedly have more than 3000 mAh/g of capacity and adequate initial efficiency of 96%.
However, the volumetric expansion by the reaction with Li changes the length (or thickness) of the columnar structures as well as the width of the columnar structures from 6 μm to 17 μm, and then reversibly changes to 11 μm. Such problems in the efficiency of electrode processes forming the columnar structures exist, as well as in Li concentration gradient in portions of the columnar structure and in forming second products with the current collector formed of copper (Cu). Meanwhile, research is on-going with regard to methods of controlling the structural forms such as micro-porous, nanofiber, tube, rod, and the like. However, the applicability of such methods is uncertain.
Meanwhile, in case of using a Titanium (Ti)-based material such as lithium titanium oxide (Li4Ti5O12: LTO), LTO exhibits only minute changes in volume (0.1-0.2%) during a reaction with Li. LTO, having zero-strain in crystal lattice, performs excellently in output, extended lifespan, and stability. However, LTO also includes weaknesses in that it has higher electric potential and lower capacity than those of graphite.
Meanwhile, besides the Si and Ti materials, interest in silicon oxide (SiOx) materials is increasing. Silicon oxides have high capacity, of approximately 1000 mAh/g, but exhibit weakness in terms of volumetric expansion and in the fact that relatively expensive SiOx are used in large amounts when forming compound materials.
Therefore, in order to be applicable for use in automobiles that require high capacity, such as hybrid vehicles (HV), plug-in hybrid electric vehicles (PHEV), and the like, development of relatively low-priced SiOx-based anode materials having low volumetric expansion and stable electric potential for secondary batteries is urgent.
Meanwhile, there are related art documents 1 through 3. Patent document 1 relates to a method of forming a high-capacity anode material for a secondary battery by forming porous carbon, but patent document 1 does not disclose a technology of using SiOx as an anode material. Patent document 2 relates to a technology of coating silicon-based materials and carbon materials with an organic solvent, but patent document 2 includes a wet method having a low applicability and an inconvenient process. Patent document 3 relates to a technology with which a silicon-carbon composite is prepared using gas plasma. According to the technology, the silicon-carbon composite is prepared by activating a silicon precursor by the gas plasma on the surfaces of graphite- or coke-based carbon particles using a spray method. This technology utilizes relatively expensive plasma, thus having a low applicability.                (Patent Document 1) Korean Patent Laid-open Publication No. 2011-0053027        (Patent Document 2) Korean Patent Laid-open Publication No. 2011-0046076        (Patent Document 3) Korean Patent No. 0761000        